1. Field of the Invention
The present invention relates to a method of automatically requesting retransmission in the event of a message being badly received, otherwise known as an "ARQ method", for a duplex digital data transmission installation having at least one noisy return channel, and also to an installation enabling the method to be implemented.
2. Description of the Related Art
Three strategies are in general use for reducing the error rare of a digital transmission.
The first strategy consists in using an error-correcting code without a return channel (known as "forward error control" or "FEC"). A drawback of this error correction strategy is its poor adaptability which can be a handicap with certain types of channel (a tropospheric channel or a channel suffering jamming). For example, with a tropospheric channel, various channel parameters such as mean signal-to-noise ratio, coherence band or coherence time remain fixed only on a small scale. If these parameters are observed over several hours, several days, or several months, it is observed that they are subject to non-negligible fluctuations. Thus, a code designed to function ideally with given parameters will probably be ineffective at certain times of the day or of the year. Although this strategy does have the advantage of ensuring a transmission delay which is constant, it is ineffective against bursts of errors unless it is associated with interlacing, which has the drawback of increasing transmission delay.
A second strategy is to make use of a return path, by using the ARQ ("Automatic Repeat request") strategy where errors are detected and retransmission is requested. This strategy may be defined as follows:
data is sent in packets, rather than continuously; PA1 each packet contains information symbols accompanied by check symbols; and PA1 the receiver makes use of the check symbols to detect errors in transmission, if any. Depending on the result of the check, the receiver either accepts a block or else requests retransmission. PA1 the receiver has a decoder which attempts to decode the sequence as received, and a specific criterion is used representative of the confidence that may be had in the decision of the decoder; and PA1 if the criterion is satisfied, then the decoded sequence is accepted as being the same as the sequence that was transmitted, otherwise a request is made for retransmission. PA1 when the receiver receives a repeat request, it does not take it into account if the requested block has been requested less than N turns previously; and PA1 when the receiver requests the same block several times in succession up to N times, only the first request is taken into account.
The ARQ strategy thus makes use of an error-detecting code, whereas the FEC strategy makes use of an error-correcting code. As a general rule, the decoding required for error-detecting codes is simpler than that required for error-correcting codes.
This strategy gives rise to additional redundancy in the form of retransmission. Thus although the redundancy specific to the encoding is constant (and given by the code ratio), the amount of additional redundancy is a random variable. The total mean redundancy is given by the effective code ratio. The main drawback of this strategy is that when transmission conditions are bad (very noisy channel), the error-detecting code is quickly submerged and the number of repeats becomes very large, thereby increasing transmission delay.
The third strategy currently in use seeks to mitigate this problem by associating the ARQ strategy with an error-correcting code.
This third strategy is referred to as a "hybrid" strategy since it combines error-correcting code with an ARQ procedure. The code used serves not only to detect errors, but also to correct them within the limit of its capability. The strategy is thus as follows:
Both the second strategy and the third strategy require retransmission to be requested. The three most conventional schemes currently in use are as follows:
The simplest scheme is "stop-and-wait" ARQ. The transmitter sends a block and then before sending the next block it waits for the receiver to acknowledge that it has received the transmitted block correctly. The main drawback of these scheme is that too much time is spent waiting for a positive or negative acknowledgement.
The second scheme is "go-back-N" ARQ and is both more complex and more efficient. Packets of symbols are transmitted on a continuous basis. Replies from the receiver arrive after a certain time delay equal to the time during which the transmitter sends a further N-1 packets. If the transmitter receives a request for retransmission (corresponding to an error in a packet), then it retransmits not only the erroneous packet, but also all the following packets. This system is well adapted to certain channels having memory where a first erroneous packet is very often followed by other packets suffering from noise. This applies to a high data rate tropospheric scatter channel.
The third scheme is the most complex but also the most efficient. It is called "selective-repeat" ARQ. As in the preceding scheme, blocks are sent on a continuous basis, and only erroneous blocks are retransmitted. The receiver must therefore be fitted with a large buffer memory to be able to reproduce the blocks in order. This is the most difficult ARQ protocol to implement.
In all of the technical literature published so far and of which we are aware, performance calculations relating to the various ARQ strategies and schemes always assume that the return channel is noise-free. A repeat request requires only a few bits at most and it can therefore be encoded in a manner that is very effective, using enormous redundancy to be sure that the request is properly received. However, such a method is not applicable when there is high rate data flow in both directions, e.g. in a duplex digital telephone installation. If the return channel is to be used for improving the quality of a link, there can be no question of significantly increasing its data rate. Only a small amount of space can be allocated to frame management, and it must be assumed that the return channel is noisy, which necessarily degrades performance since the transmitter may fail to receive a positive or negative acknowledgement from the receiver, either because the return channel is noisy on a random and non-intentional basis (as applies to a troposphere channel), or else, and above all, because the return channel is being jammed intentionally by an intelligent jammer.